1. Field of the Invention
The present invention relates to a telescopic steering column, and, in particular, to a telescopic steering column apparatus by which the total length of a steering column having a steering shaft therethrough can be shortened and lengthened, so that the position of a steering wheel provided on the end of the steering shaft can be adjusted to match the physique and driving posture of the driver of the vehicle.
2. Description of the Prior Art
Conventionally, both a tilt steering apparatus and an expansion steering column apparatus (also referred to as a telescopic steering apparatus) are commonly known as devices for adjusting the position of a steering wheel to match the physique and driving posture of a driver of a vehicle.
FIG. 1 shows a steering apparatus which comprises both the above-mentioned types of tilt steering mechanism and expansion steering column mechanism incorporated therein.
The tilt steering mechanism of the steering apparatus in FIG. 1 is provided by a steering shaft 3 inserted through a steering column 2. One end section (the right end in FIG. 1) of the steering shaft 3 is displaced in the direction of the arrow a in FIG. 1 by the operation of an adjustment lever 1 for adjusting the height of a steering wheel (omitted from the drawing) secured to the end section of the steering shaft 3.
In addition, the expansion steering mechanism of the steering apparatus in FIG. 1 is provided by the steering column 2 which combines an outer column 5 and an inner column 6 in a telescopic form. The steering column 2 can be extended or retracted to change its total length by operating an adjustment lever 4. One end of the telescopic steering shaft 3 is displaced in the direction indicated by the arrow b in FIG. 1, to adjust the longitudinal position of the steering wheel secured to the end section of the steering shaft 3.
Since the structure and operation of a typical tilt steering apparatus is disclosed in detail in Japanese Laid Open patent application No. 60-144569, and not essential for the present invention, further details are omitted in the present specification.
The construction of a conventional expansion steering column apparatus is illustrated in FIG. 2 and FIG. 3.
The steering column 2 comprises a cylindrical outer column 5 and a cylindrical inner column 6 combined in a telescopic form. Inside the inner column 6, the middle section of the steering shaft 3 is supported by a pair of deep-groove-type (or angular-type) ball bearings 7. The midsection of the steering shaft 3 is provided with a serrated engaging section 8, and the steering shaft 3 is extended or retracted by the telescopic action of the steering column 2.
A lock housing 9 is projected from the outer column 5 in a lateral direction (downward in FIG. 2, to the left in FIG. 3) and secured to part of the midsection of the outer column 5. A cylindrical space 11 for the insertion of a lock member 10 is provided in the lock housing 9. The lock member 10 is detailed later.
The cylindrical space 11, which is shaped as a rectangular parallelopiped, are defined at the two sides (upper and lower sides in FIG. 3) thereof by a pair of plane surfaces 17 provided with a space therebetween and parallel to the center axis of the outer column 5. A communicating opening 12 is formed through the side wall of the outer column 5 in alignment with the cylindrical space 11 to provide communication between the cylindrical space 11 and the interior of the outer column 5. The lock member 10 is inserted into the cylindrical space 11 through the opening 12, and the inner surface of the lock member 10 is adapted to abut the outer peripheral surface of the inner column 6 which axially penetrates the inside of the outer column 5.
The inside surface 13 of the lock member 10 is formed in a concave arc-shape with almost the same curvature as that of the outer peripheral surface of the inner column 6. The relative displacement of the inner column 6 and the outer column 5 can be prevented by the application of strong pressure through the arc-shaped concave surface 13 against the outer peripheral surface of the inner column 6.
A threaded hole 15 is formed in the center of a partition 14 which defines the outer side of the cylindrical space 11. The inner end of a lock screw 16 which is screwed into the threaded hole 15 abuts the center section of the outer surface of the lock member 10. The base of the adjustment lever 4 is secured to the outer end of the lock member 10. The arc-shaped concave surface 13 can be pressed against the outer peripheral surface of the inner column 6 by the operation of the adjustment lever 4.
In addition, a guide pin 18 is secured to the bottom of the lock housing 9, and penetrates the lower wall of the outer column 5. The guide pin 18 engages an elongated slot 19 which is formed in an axial direction in the lower wall of the inner column 6 so that the inner column 6 is prevented from rotating with axial movement only.
In the expansion steering column device with the above-described configuration, the adjustment of the full length of the steering column 2 is performed in the following manner.
In making the adjustment, first the lock screw 16 is retracted (to the left in FIG.3) by the operation of the adjustment lever 4. As a result, the force of the lock screw 16 pressing the lock member 10 against the outer peripheral surface of the inner column 6 is released, so that the inner column 6 can be freely displaced inside the outer column 5.
In this state, the steering wheel (omitted from the drawing) secured to the end of the steering shaft 3 is pushed or pulled to adjust its longitudinal position. As the steering wheel is pushed or pulled, the serrated engaging section 8 provided in a mid section of the steering shaft 3 is displaced, and the inner column 6 provided on the outside of the steering shaft 3 and supported by the ball bearings 7 is displaced longitudinally.
As a result, once the longitudinal position of the steering wheel has been adjusted, the lock screw 16 is advanced (moved to the right in FIG. 3) by operating the adjustment lever 4. The advancing lock screw 16 presses the lock member 10 against the outer surface of the inner column 6. This causes a strong frictional force between the arc-shaped concave surface 13 formed on the inner surface of the lock member 10 and the outer surface of the inner column 6, so that the inner column 6 is maintained on the inside of the outer column 5 with no possibility of displacement, whereby the steering wheel is maintained at the adjusted position.
However, in the operation of a conventional expansion steering column apparatus with this configuration, a large operating force is necessary to pivot the adjustment lever 4 when securing the inner column 6, because of the friction and the like in the screw section. If the operating force is reduce, the adjustment lever 4 must be pivoted through a large angle. In addition, during the operation, the person operating the lever does not have a good feel for the operation because there is no way of sensing the good feeling of adjustment.
After a specified torque has been applied to the lock screw 16, the lock screw 16 and the adjustment lever 4 are secured. In the case where the lock screw 16 and the lever member 4 are conventionally connected together by a serrated linkage, the adjustment of the positional relationship of the two members 4, 16 can only be done by one pitch of the serrations at a time, so that fine adjustment of this positional relationship is difficult.
Other examples of the structure of an expansion steering apparatus are disclosed in Japanese Laid Open patent application No. 16-145070 and No. 63-152771, but the same type of problems are found in these apparatus.
In Japanese Laid Open patent application No. 61-220965, an expansion steering column apparatus using a toggle mechanism to secure the inside column to the inside of the outside column is disclosed. However, there are some problems, e.g. the securing of the inner column is unreliable because pressure is applied at one point only on the inner column, or the wall thickness of the inner column must be very thick in order to prevent distortion of the inner column along with the securing operation.
Specifically, in the case where the inner column is secured to the inside of the outer column, a lock member is firmly pressed against the outer peripheral surface of the inner column, and since there is only one pressure point, when this lock member faces the outer peripheral surface of the inner column, a high pressure must be applied to make the securing of the inner column reliable.
With this type of high pressure, an adequate rigidity must be ensured by making the inner column thick-walled so that the inner column is not distorted. This causes the manufacturing costs to rise drastically and the weight to increase.